Conventional drum closure systems require a plug to close the drum openings during drum painting. This is to prevent the paint from contaminating the inside of the drum. Generally, drum openings are of two sizes, 20 mm and 50 mm. Therefore, flanges and plugs also generally come in 20 mm and 50 mm sizes. Conventional plugs are generally of 1.0 to 1.2 mm thickness and capseals are generally of 0.3 mm thickness. The conventional plug cannot be fully torqued into the flange threads before painting because at the time of paint curing which happens typically at 180 degrees centigrade, the hot air inside the drum needs to be vented, failing which pressure builds up inside the drum. This can lead to deformation of the drum. Therefore, conventional plugs need to be partially screwed into the flange to provide an outlet for the hot air during paint curing. However, the disadvantage of screwing the conventional plug partially into the flange is that paint spray shall fall on plug gasket and some part of flange thread chamfer which is unacceptable to drum customers who fill critical chemicals.
For this reason, flange openings of the drum in conventional systems are more often than not closed with a paint plug just before the paint operation. The function of the paint plug is to cover the flange opening to prevent paint ingress inside the drum and on the flange inner wall. At the same time, the paint plug allows the escape of hot air from the drum during paint curing, preventing pressure build-up inside the drum. The disadvantage of using a paint plug besides the cost of material is that the process of insertion of the paint plug at present is generally a manual operation.
The paint plug typically is taken off after paint curing either manually or by a magnetic conveyor running overhead. However, often the magnetic conveyor fails to take off the paint plug. Therefore, another disadvantage of using the paint plug is that removal cannot be fully automated and requires manual supervision.
A paint plug can be re-used a number of times before a thick layer of paint builds up on it. For environmental reasons, the paint plug has to be stripped of paint before putting it back to use or discarding it after a number of uses. Stripping of the paint plug also involves additional costs for the drum manufacturer. Further, paint stripping needs to meet strict environment standards, for which costs are higher.
To save on the cost of painting stripping, some drum manufacturers with lower quality requirements may avoid the use of a paint plug and instead directly insert a conventional plug loosely into the flange opening to allow for the passage of hot air from inside the drum during paint curing. This does not prevent some paint from coming in contact with the plug gaskets and the flange inner wall during paint spraying. Therefore, when drum manufacturers save on the cost of the paint plug, it results in a compromise on quality.
The conventional plug insertion is not generally automated and has to be done manually. Auto insertion of plugs is easier when insertion is done on the drum head immediately after flanges are inserted as compared to insertion of plug after the drum head is assembled with the drum body. When the insertion in on the drum head prior to drum assembly, the drum head is automatically handled, angular position of the drum head opening is known and alignments needed to correctly find the threads are easy to obtain as the drum head is still a flat sheet. However due to the need for maintaining a passage for air to escape during paint cure, conventional plugs cannot be torqued. Paint plugs generally do not even have threads to torque and a loose plug would get dislodged in drum assembly. Therefore conventional plug or paint plug insertion before drum painting has not generally been automated in industry.
Paint curing is followed by the leak detection process which may be carried out generally by helium injection into the drum. Nitrogen is generally injected for corrosion control in the drum interior. The paint plug is taken off after paint curing to be replaced with a conventional plug after nitrogen and helium injection.
Where conventional plugs have been used instead of paint plugs before the paint operation, generally the larger 50 mm plug is removed to inject nitrogen and helium and then both 20 mm and 50 mm plugs are torqued in and the openings sealed. Thereafter, the drum goes through a helium tester to detect leaks after which it can be shipped to the customer
After leak testing the drum is sent for filling which is generally carried out by unscrewing the 50 mm plug, filling the drum and re-torqueing the plug. A capseal is fed over both the 20 mm and 50 mm plugs. Before shipping to the customer, the capseals are crimped. The role of the capseal is to make the drum tamperproof. The capseal is so designed that it needs to be destroyed in order to detach it. Separate equipment is required to feed and crimp the capseals. The hybrid plug has been designed to be used in conventional drum closure system as a replacement of the conventional plug, paint plug and capseal and offers many advantages as described below.